As the Fox alternate-universe drama “Fringe” ends its five-season run, let us pause to praise John Noble’s textured performance as Walter Bishop — and his talented potrayals of Walter Bishop, Walter Bishop and Walter Bishop. And Anna Torv? Her work as Olivia Dunham, Olivia Dunham and Olivia Dunham has built a one-dimensional character into a genuine, multifaceted sci-fi heroine.

This is the situation in which “Fringe” fans find themselves ahead of the final, 2-hour conclusion that airs 8 p.m. Jan. 18. So what on Earth — or, given that it’s “Fringe,” what on Earths — are we talking about here?

Only the fact that, unlike any other show in recent memory — or, perhaps, in television history itself — “Fringe” has required something of its troupe of actors that is both daunting and utterly captivating to watch: It forced them to play several different versions of their characters, sometimes all at once, and define unique characteristics and emotional memories for each one over time.

This has been the case since the first season, which introduced the notion of multiple universes with a finale called “There’s More Than One of Everything.” By and by, the multiplicity produced dialogue you wouldn’t find anywhere else, like the time when Noble’s Walter Bishop says to Jasika Nicole’s Agent Astrid Farnsworth: “You’re not you, are you?”

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For the past five seasons, “Fringe” has chronicled the exploits of Olivia, forced to collaborate with licorice-chomping, soft-hearted, guilt-ridden mad scientist Dr. Bishop to explore “fringe events,” weird occurrences that suggest a nefarious plan is afoot to threaten the world. Joining them is Walter’s adult son Peter (Joshua Jackson), who has played a special role in all that’s unfolding.

Jackson, though, ended up the only main character in the show who wasn’t regularly playing different versions of himself.

Actors playing multiple roles isn’t new. Alec Guinness did it in the 1949 movie “Kind Hearts and Coronets,” in which he portrayed a whopping eight roles, including a woman. But those were all different characters. Soap operas have been replete with people undergoing plastic surgery to resemble others and evil agents replacing good characters. Mike Myers, of course, perfected the comedic version of this in the “Austin Powers” movies, in which he plays multiple characters including the title role and the villain, “Dr. Evil.” Comedians from Jerry Lewis to Eddie Murphy to Tyler Perry have both prospered and flopped while dabbling in this kind of duality.

Rarely, though, is a performer called upon to develop the same recurring character in two similar worlds. Leonard Nimoy did it with a memorable turn as a goateed, ironfisted Mr. Spock in an original “Star Trek” episode that sent the Enterprise crew into a more bloodthirsty parallel universe. (No coincidence, perhaps, that a chunk of the team that makes “Fringe” is also behind the new “Trek” movies.)

“Fringe” was an unusual acting challenge for sure, largely because of its slow burn. The show has spent five seasons engineering crossovers to alternate universes, rebooting universes entirely and generally exploiting the entertainment potential of quantum theory to produce slight variations on characters who were subtly different based on the experiences their circumstances forced them to endure.

Thus Blair Brown has played her character, Nina Sharp, as a frosty tycoon, a mysterious ally and even a maternal figure to a main character. Seth Gabel developed two versions of Agent Lincoln Lee in two separate universes — one quiet, earnest and slightly shy, the other gregarious, trash-talking and militaristic. Fringe Division’s leader, Phillip Broyles (Lance Reddick), is a supervising agent in one reality and a colonel in another — kind but cold in the first, cold but kind in the second. And in one universe, Agent Farnsworth is a smiling, empathetic foil to Walter Bishop; in another, she’s colder, numbers-focused and shows signs of Asperger’s.

But the most fascinating permutations of this have come from Torv and Noble, two of the three main characters around whom the show’s own universe spins.

Torv’s initial Olivia Dunham was reserved, emotionally stunted, unwilling to trust or commit — based partially on her experiences with an abusive stepfather. The alternate-universe “Fauxlivia” was tougher, more disciplined — and more outgoing and willing to take what is hers. And the Olivia of later seasons, a hybrid of the original and a third one, is still reserved but able to access her feelings more readily. (In one story, Torv even portrayed a version of herself as possessed by Leonard Nimoy’s consciousness. Don’t ask.)

Noble’s performances, though, have been the show’s tour de force. He has played Dr. Walter Bishop as an addled old man, a misguided genius, a ruthless technocrat and a combination of all three — plus a younger version of himselves. What’s more, he played “Walternate,” the alternate-universe defense secretary whose motives are shadowy but whose methods are downright cruel. Some of the show’s most electric moments unfolded when Noble shared screen time with himself — not because of the differences in the characters but because of their similarities even under very different circumstances.

And that’s the point, really.

Science fiction though it is, “Fringe” has always been about universal themes: family, responsibility to community and, in the end, how our experiences shape our identities. Because even in our own universe, aren’t we all different people — each of us, every day, calibrating our identities slightly to fit an ever more complex web of moments and interactions? That’s what “Fringe” did. It examined how to be a father when you’re also a scientist, how to be human when you’re also very alien, how to be a parent when you’re also a child. Just like us, although we don’t have doppelgangers in alternate universes to compare ourselves to.

“Fringe” dug into the very unplugged notion that we all contain multitudes — that while our identities contain certain core components, the challenges and triumphs and tragedies we face can propel us in utterly different directions that sometimes even we don’t recognize.

“Must be nice to know who you are, to know your place in the world,” Peter Bishop says in one episode. But in “Fringe,” no one was ever really certain. And the show concludes with that powerful message: There but for the grace of God go I. And I and I and I and I.